In conventional examples, at an outer peripheral portion of an SOI wafer fabricated by the ion implantation delamination method, an SOI layer is not transferred, and a terrace where a base wafer surface is exposed is formed. A main cause thereof is that bonding force between bonded wafers is weak since flatness of the wafers becomes poor due to polishing sag at a wafer outer peripheral portion and the SOI layer is hardly transferred to the base wafer side.
Observing a terrace portion of this SOI wafer with the use of an optical microscope, an SOI island that the SOI layer is isolated in an island shape is observed at a boundary between the SOI layer and the terrace portion. It can be considered that the SOI island is generated in a transitional region of flatness that allows transfer of the SOI layer and flatness that does not allow transfer of the same. There is concern that such an SOI island delaminates from a wafer during a device fabrication process, turns to a silicon particle to again adhere to a device fabrication region, and causes a device failure (Patent Literature 1).
Further, in the ion implantation delamination method, since a width of the terrace portion (a terrace width) is determined based on flatness of the terrace portion of a wafer to be bonded, controlling the terrace width after bonding is difficult. For example, in case of creating a laser mark or the like on the terrace portion of the SOI wafer in a device process, there is concern that the laser mark cannot be created when the terrace width is too small.
As a method for improving such an SOI island and controlling a terrace width, there is a method for immersing a wafer subjected to bonding in an HF containing aqueous solution and etching an insulator film placed between a bond wafer and a base wafer from an outer periphery (Patent Literature 2).